1. Field of the Invention
The present invention relates to metallic prosthesis parts with layers of bio-active material, and to the use of such prosthesis parts for replacing animal and human bone.
2. Description of the Prior Art
Abnormal changes of and accidental injuries to bones and joints are remedied and eliminated at present by replacing the defective and damaged parts by prostheses made of inanimate materials which are foreign to the body, such as, for instance, metals, metal alloys, and plastic materials.
Recently, however, it has been found that metallic prosthesis parts which are anchored within the bone by means of plastic cements or adhesives, become loose after a certain period of time. Furthermore, frequently corrosion of the implanted materials has been observed. As a result thereof, for instance, hip-joint prosthesis are implanted, at present, almost exclusively in elderly patients only because implantation surgery in younger patients has to be repeated from time to time.
According to German Auslegeschrift (Application published for opposition) No. 23 06 552 corrosion can be avoided by selecting a suitable implant material or by coating the metallic prosthesis parts. However, even when proceeding in this manner, the prosthesis parts which are anchored within the bone still become loose. Thus, there is a need for a new implant material which adheres firmly to its implant bed without requiring anchoring by means of a bone cement.
It was believed that the oxidic ceramic materials (Al.sub.2 O.sub.3) of high strength properties represent such materials. However, the assumption that such oxidic ceramic materials grow readily together with the live bone has not proved to be true. Although bones and tissue grow and adhere to the implant material and although bone substance is found also in the hollow spaces of the pores of an implant material having a porous surface, there has not been achieved a true complete union between bone and implanted prosthesis.
Although the oxidic ceramic materials possess no bio-activity, they represent an important raw material for producing implant parts. This is due to some of their properties such as, for instance, their high stability with respect to corrosive effects in the human organism and their high resistance to abrasion. This latter property represents an important parameter for the construction of moving parts of the body such as its joints. A joint implant which consists of a combination of metal or, respectively, metal alloy, which forms the shaft of the prosthesis, and of oxidic ceramic material which forms the ball and the socket of the joint has been disclosed in German Offenlegungsschrift (Published Application) No. 21 34 316. However, both said prosthesis parts are not bio-active and thus render it rather difficult to effect implantation without the use of a bone cement.
Recently new materials which enter into intimate and firm connection with living bone tissue on account of their chemical composition and structure have been developed. But these bio-active materials, for instance, bio-active glass ceramic material and bio-glass, do not have sufficiently high mechanical strength properties, so that it is not possible to produce therefrom load-carrying prosthesis parts. When coating the heretofore known implant materials with such bio-active agents, it is possible to firmly anchor the same in the bone tissue without the additional use of bone cement.
The objective is to provide a prosthesis which is both bio-active and possesses high load-carrying capacity, particularly for an indefinite period of time. With respect to the latter requirement, analyses of the movement and jumping action of the human body have shown that, under specific loads, pressure and stress peaks of seven times the body weight can occur. If it is desired to replace, for example, the femur by an endoprosthesis, it is necessary based upon anatomical consideration for this replacement piece to conform essentially to the form, size and configuration of the natural skeleton portion being replaced. This means that the endoprosthesis cannot be dimensioned cross-sectionally larger than its natural predecessor at the critical locations at which it is subjected to these enormous loads.
Practically speaking, in order to provide a prosthesis part having the required strength properties, it is necessary to employ a base member which is made of metal or a metal alloy. The basic problem to be solved, therefore, is the anchoring of the bio-active material to the base member. The layer of bio-active material must be permanently attached to the base member, so that it does not separate at any time after implantation in a patient's body.
Applying a layer of bio-active material to the prosthesis material according to known processes such as by spraying by the flame-spraying process or the plasma-spraying process, by dipping or by other methods, does not yield implant material of the required firmness of attachment and freedom from pores.
Furthermore, when the prosthesis material is coated in such a manner, there are encountered temperatures which, for instance, in the case of bio-active glass ceramic materials, partly destroy the crystallites which are responsible for growing together of the implant material and the bone tissue. To overcome this disadvantage, it was necessary to effect recrystallization of the glass ceramic material. For this purpose the coated implant part had again to be subjected to a heat treatment. However, temperatures of about 900 degrees C. as they are required to effect recrystallization, negatively affect the properties of metallic implant parts.